In recent years, due to its light weight and low power consumption, a liquid crystal display device has been used in a variety of fields. Specifically, the liquid crystal display device, for example, is used as (1) a display device such as an office automation equipment (for example, word processor and personal computer), and an audio and visual equipment dealing with an image, (2) an information display device for use in (a) instruments of a car and an air plane etc. and (b) portable information terminals, and (3) an information display device for use in a watch and a measuring device.
Generally, the liquid crystal display device itself is non-light emitting. Thus, by providing the liquid crystal display device with illuminating means of some sort or means serving the same function, it is possible to adopt the liquid crystal display device as a display which is available in various forms. The liquid crystal display device can be broadly classified into three types on the basis of the difference in illuminating methods: a transmitting type, a reflecting type, and a semitransmitting type.
A transmitting-type liquid crystal display device is provided with illuminating means such as (1) a small fluorescent lamp, e.g., a cool cathode fluorescent tube, and (2) an EL (electroluminescence) light-emitting element. Such illuminating means is provided on the back surface of a liquid crystal display element, namely, the illuminating means are provided on a surface opposite to the displaying surface of the liquid crystal display device. With this arrangement, the transmittance of the light from the illuminating means is adjusted by the liquid crystal display element such that the light is transmitted to the front surface of a display panel, thereby displaying an image on the displaying surface of the liquid crystal display element. Thus, since the liquid crystal display device is provided with illuminating means, such liquid crystal display device can be used in a dark environment in the same manner as a self-light-emitting element. Further, by controlling the intensity of the light from the illuminating means, it is possible to adjust the brightness of the display screen.
However, in the liquid crystal display device of this type, even when used in a luminous environment, the illuminating means is required to carry out illumination with certain intensity. For this reason, even though the liquid crystal display device is, in general, said to have low power consumption, the power consumption is increased. Also, because it is required to provide a power source for supplying power to the illuminating means, the liquid crystal display device has a drawback in that the liquid crystal display device becomes large and heavy.
On the other hand, a reflective-type liquid crystal display device uses, as reflecting illumination, surrounding light such as the sun light and indoor illumination which are available in the environment where the reflective-type liquid crystal display device is used. Hence, the reflective-type liquid crystal display device is provided with reflecting means for reflecting the surrounding light. The reflecting means is provided on the back surface of the liquid crystal display element. Note that, the reflecting means is composed of, for example, (1) aluminium (Al) or Silver (Ag) and (2) a film substrate. Thus, in the reflective-type liquid crystal display device, unlike the transmitting-type liquid crystal display device which is required to be provided with illuminating means, it is not required to provide illuminating means. As a result, in the reflective-type liquid crystal display device, it is possible to lower the power consumption, also it is possible to realize a light, thin liquid crystal display device with ease which is a characteristic of a liquid crystal display device.
However, since the reflective-type liquid crystal display device uses the surrounding light, the brightness of the display is largely influenced by the lighting condition of the environment where the reflective-type liquid crystal display device is used. That is to say, for example, when the reflective-type liquid crystal display device is used in a dark environment, it becomes difficult to see the display. For this reason, the reflective-type liquid crystal display device is not suitable in a dark environment.
In contrast, a semitransmitting-type liquid crystal display device, like the transmitting-type, is provided with illuminating means such as (1) the small fluorescent lamp, e.g. the cool cathode fluorescent tube, and (2) an EL light-emitting element. The illuminating means is provided on the back surface of the liquid crystal display element, namely, the illuminating means is provided on a surface opposite to the displaying surface of the liquid crystal display element. Between the illuminating means and the liquid crystal display element, there is provided a semitransparent reflecting plate such as a magic mirror. The main portion of the semitransparent reflecting plate is made of (a) a scattering plate such as a semitransparent plastic sheet, (b) a scattering plate in which a meshed metal reflecting film has been patterned on the scattering plate of (a), or (c) a scattering plate on which pearl resin etc. has been spread.
With this arrangement, in a luminous environment, displaying is carried out without lighting of the illuminating means, but by the reflection of surrounding light such as indoor illumination by the semi-reflecting plate. Namely, in this case, the surrounding light is used as reflecting illumination. Note that, displaying without lighting of the illuminating means in this manner will be referred to as a reflecting mode hereinafter.
In contrast, in a dark environment, displaying is carried out by lighting of the illuminating means. Namely, in this case, the light from the illuminating means is used which transmits through the semitransmitting reflecting plate. Note that, displaying by lighting of the illuminating means in this manner will be referred to as a transmitting mode hereinafter.
As described, since the liquid crystal display device of this type is provided with both the transmitting type and the reflecting type displaying systems, a suitable displaying system can be adopted in accordance with a lighting environment so as to complement each other the shortcoming of each displaying system. Namely, the semitransmitting-type liquid crystal display device has an advantage in that the power consumption is low, and the device is suited for use not only in a luminous environment but also in a dark environment.
Incidentally, the characteristic of the semitransmitting-type liquid crystal display device is defined fixedly by setting of the transmittance and the reflectance of a material of the semitransmitting reflecting plate. That is to say, in the case of using, for example, a semitransmitting reflecting plate which has been set so as to have high transmittance and low reflectance, the illuminating characteristic of the liquid crystal display device becomes transmission-oriented. On the other hand, in the case of using, for example, a semitransmitting reflecting plate which has been set so as to have low transmittance and high reflectance, the illuminating characteristic of the liquid crystal display device becomes reflection-oriented. Note that, in the semitransmitting-type liquid crystal display device, ignoring the light loss due to scattering in the semitransmitting reflecting plate, the transmittance and the reflectance substantially trade off with each other. Recently, a semitransmitting-type liquid crystal display device which is highly transmitting as well as highly reflecting has been developed; however, the transmittance and the reflectance still trade off with each other.
A semitransmitting-type liquid crystal display device, for example, adopting the STN (Super Twisted Nematic) system is designed, considering the visibility in both the transmitting mode and the reflecting mode, so that the respective contrast in the transmitting mode and in the reflecting mode are substantially the same. Therefore, in the semitransmitting-type liquid crystal display device adopting the STN system, both the transmitting type and the reflecting type displaying systems can be adopted. Note that, the contrast refers to the ratio of respective luminance values when displaying white and when displaying black, while driving the liquid crystal display element.
However, in the semitransmitting-type liquid crystal display device adopting the STN system, due to its arrangement, if a large number of scanning lines are employed, a charge stored during a period of (a) applying a voltage to two transparent electrodes sandwitching the liquid crystal layer and (b) applying the next voltage to the two transparent electrodes is discharged so as to lower the contrast. Also, the contrast in the transmitting mode is lowered compared with, for example, the transmitting-type liquid crystal display device or the reflecting type liquid crystal display device. This presents a problem in that the displaying becomes dark in the reflecting mode and pale in the transmitting mode such that the visibility of the displayed image becomes poor.